Two nematode species, Bursaphelenchus xylophilus and B.mucronatus, are transmitted by two cerambycid species, Monochamus alternatus and M.saltuarius, in Japan. Transmission occurs via feeding and oviposition wounds of insect vectors. Mathematical model of transmission via two pathways was constructed and the rate of transmission was estimated in forests. The estimate was 0.11 for avirulent B.mucronatus while 0.0062 for virulent B.xylophilus. B.mucronatus was shown to move between M.saltuarius during its mating behavior. The nematode transmission from male to female vectors occurred easier than the reversed case. The nematodes moving from male to female vectors were shown to be transmitted to pine bolts via the oviposition wounds. When B.mucronatus entered the pinebolts via M.saltuarius oviposition wounds, it remained within 0.75 cm of the ovipositor-inserted holes at least for 24 hours. In the B.mucronatus-M.saltuarius system, 17 % of nematodes which had been in the vector bodies at emergence were transmitted via the oviposition wounds and 6 % via the feeding wounds. In the B.xylophilus-M.alternatus system, 3 % of nematodes present in newly-emerged vectors were transmitted via the oviposition wounds and 12 % via the feeding wounds.